1. Field of the Invention
The present invention relates to a handoff control method in a mobile communication system and a mobile station implementing thereof.
2. Description of the Related Art
In recent years, services which distribute, through distribution to a plurality of users, contents such as images and voices via a wired broadband line, such as ADSL and FTTH, are becoming increasingly popular. IP multicast technology, represented by Internet Group Management Protocol (IGMP), has implemented such a multicast service.
On the other hand, with the diffusion and sophistication of mobile phones, expectations for wireless multicast service provisions are rising. Specific methods for providing multicast service using mobile phones have been proposed (for example, refer to Japanese Patent Laid-open Publication 2001-308856). In the above mentioned technology, each cell has a forward traffic channel provided for multicast services, and all mobile devices using the service monitor this same channel. In addition, when the mobile device moves between cells while using the multicast service, a normal idle handoff processing is performed, after which reception of multicast service is performed once again in the cell of the destination.
Automobile/mobile phone systems take on a so-called cell configuration, wherein wireless zones are divided finely in order to effectively use limited frequency segments and repeatedly implement the frequency for every cell of a certain interval.
Handoff (handover) refers to control which changes connection from the base station to which connection is currently made to the base station of the cell of the destination. For example, horizontal handoff is a handoff between the same wireless systems and a vertical handoff is handoff between different wireless systems.
Idle handoff control refers to control for changing the base station connection when in an idle state.
Here, common idle handoff methods are as follows:    (1) a mobile station which is in an idle state in a first cell moves to a second cell.    (2) the strength of the pilot signal of the first cell gradually falls and the strength of the pilot signal of the second cell gradually rises.    (3) idle handoff processing begins in the second cell when the difference in the strength of the pilot signal of the second cell and the strength of the pilot signal of the first cell exceeds a certain threshold value.
Here, the foregoing threshold value can be, for example, a predetermined fixed value. However, although, on one hand, the idle handoff plays a part in actualizing a favorable incoming rate by actively catching pilot signals with high pilot strengths, there is a problem in that performing frequent idle handoff is not desirable from the point of view of power consumption.
Therefore, a method is proposed for controlling the increase in idle handoff frequency due to sudden fluctuations in the electromagnetic wave environment by enabling adjustment according to the pilot signal strength (for example, refer to Japanese Patent Laid-open Publication 11-113042) or by implementing, not the instantaneous value of the pilot signal pilot strength, but the temporal average value (for example, refer to Japanese Patent Laid-open Publication 6-335051) However, actively catching pilot signals with high pilot strengths is not necessarily preferable while using multicast service because the multicast contents distributed to each cell differ. For example, the multicast contents distributed in the first cell is not distributed in the second cell (cannot be distributed). In this case, if reception is moved from the first cell to the second cell or remains in the vicinity of the border between the first cell and the second cell, while receiving multicast contents, the pilot signal of the second cell may become stronger and idle handoff may be performed from the first cell to the second cell. If idle handoff is performed to the second cell, the multicast contents are not distributed in the second cell, and therefore, the reception cannot be continued. In this case, depending on the user, it is preferable that reception of multicast contents is continued, even if reception conditions are poor to a certain extent. Alternatively, it is preferable that idle handoff to the base station distributing the multicast contents is facilitated and that to other base stations is made difficult.